Mammalian and chicken-II forms of gonadotropin-releasing hormone (mGnRH and cGnRH-II, respectively) have been measured simultaneously in the brain, pituitary, and peripheral terminal nerves (nasal area tissue) of adult males and females of a representative amphibian, Rana esculenta, during the annual reproductive cycle. Whereas in the male, brain concentrations of both GnRH forms showed significant reproductive status-related fluctuations, in the female brain only cGnRH-II content showed significant changes. The highest GnRH levels were recorded just prior to breeding in both sexes. In the pituitary both GnRHs were present in all seasons. In the peripheral terminal nerves, instead, only mGnRH was detected in all seasons confirming our previous immunohistochemical data. In both sexes furthermore, the brain and pituitary mGnRH levels were consistently much higher than those of cGnRH-II and there were no sex-related differences in the brain and pituitary content of GnRHs. Seasonal changes in brain GnRH levels may correlate with plasma sex steroid levels reinforcing the postulate that sex steroids affect GnRH neuronal systems.